XDJ-RX sound problem on mac

Hello,

when i connect the xdj-rx to my imac (os 10.11 el captain) via usb everything is working fine so far. i can select the xdj-rx in sound and use it as an external soundcard. but after a short moment the sound stops.

i updated to the latest drivers and firmware, but nothing works. any advices?

this process works on my macbook air (also os 10.11 el captain) perfectly well.